He portrayed Franklin D. Roosevelt in the HBO film Into the Storm (2009) for which he received a nomination for the Primetime Emmy Award for Outstanding Supporting Actor in a Limited Series or Movie.

From 2010 to 2024, he co-starred as the patriarch Henry Reagan, NYPD Police Commissioner (retired), in the multi-generational CBS series Blue Bloods.

[4] Cariou made his first appearance in Damn Yankees at Rainbow Stage in Winnipeg in 1959, and was a founding member of the Manitoba Theatre Centre.

[5] He was offered a scholarship at the National Theatre School of Canada in Montreal but, married with a young child and financial responsibilities, he was forced to decline the honor.

Six years later he won both the Tony and Drama Desk Award for his portrayal of Sweeney Todd: The Demon Barber of Fleet Street in the Stephen Sondheim musical opposite Angela Lansbury.

Cariou's early film credits include One Man (1977), and the 1977 Harold Prince-directed screen adaptation of A Little Night Music with Elizabeth Taylor.

In 1981 Cariou starred in the Alan Alda directed comedy-drama The Four Seasons alongside Carol Burnett, Rita Moreno, and Sandy Dennis.

During these years, Cariou also appeared in a number of benefits, including A Christmas Carol for the Riverside Shakespeare Company in New York, playing Scrooge, with Helen Hayes, Raul Julia, and Mary Elizabeth Mastrantonio, directed by W. Stuart McDowell at the Symphony Space in 1985.

In 1984, having directed "Death of a Salesman" at the Citadel Theater, Edmonton, he played the lead in "King Lear" and was appointed associate director.

From 1985 to 1992, Cariou appeared in multiple episodes of the popular television mystery series Murder, She Wrote with his friend and former Sweeney Todd co-star Angela Lansbury.

Cariou portrayed the recurring character of Michael Hagarty, characterized as an Irish international man of mystery who worked as a secret agent for British MI-6.

His off-Broadway appearances include Master Class, Papa (an Ernest Hemingway one-man show) and Mountain (Justice William O. Douglas).

In 2010, Cariou appeared as Madoff-like Ponzi scheme man Louis Tobin in the drama Damages, the main antagonist in season three.

He played an American naval officer in 2013's den Orolige Mannen (the Troubled Man), adapted from one of Henning Mankell's Kurt Wallander novels.